APS for Synchronous Optical NETwork (SONET)/Synchronous Digital Hierarchy (SDH) technology according to the GR-253-CORE specification entitled “Synchronous Optical Network (SONET) Transport Systems: Common Generic Criteria”, for example, requires electrical bridging between components which support redundant network-side SONET/SDH connections to subtending equipment. Electrical bridging at the physical layer provides for transmission of the same SONET/SDH payloads on redundant network-side optical connections. In a 1+1 linear APS implementation, traffic is bridged from a working connection to a protection connection at transmitting equipment, and receiving equipment normally selects traffic from the working connection. A fault or other condition affecting the working connection causes an APS operation, and traffic is then selected from the protection connection.
Physical layer bridging tends to be implemented in a single equipment chassis. This can significantly limit the actual level of protection that is provided, in that a failure affecting one equipment installation would interrupt traffic flow.
Such bridging also provides protection only for the optical SONET/SDH connections, and not for further connections, such as access-side connections from which traffic for SONET/SDH connections originates.